If you specify something otherwise, you are clearly stating you don't want to use the defaults, so if the driver says duplex and printer states simplex is the default, the driver is going to win.

Some drivers will (or at least should) pull its defaults from the printers webpage, but in my experience, this is really only the case in specific scenarios. It almost always works with a native model specific v3 Windows print driver, but almost never if using the Xerox Global Print driver. And of course the driver would need to be installed with full bi-directional comm, or it won't / can't pull those defaults.

It also isn't guaranteed to work.

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